In-situ observation of phase transformation during heat treatment process of high-carbon bainitic bearing steel

Phase transformation during austenitizing process and heat treatment process under slow cooling of high-carbon bainitic bearing steel is still unclear. Direct nano-scale microscopic observation and in-situ X-ray diffraction investigations have been used to conduct in-situ investigates of phase transformation of the tested steel, including austenitizing and two-step isothermal process under slow cooling. Results show that the austenitizing process of the sample mainly includes the formation of austenite and dissolution of cementite. Rapid phase transformation stage during the austenitizing process occurs, and it is caused by the diffusion rate of alloying and carbon atoms at different temperatures. The martensitic and bainitic transformation can occur during the low-temperature two-step isothermal transformation. Moreover, the initial bainitic transformation rate greatly accelerates due to the introduction of preformed phase. The evolution of supercooled austenite have been explored to further understand the phase transformation behavior. The analysis of diffraction peak of undercooled austenite shows that the asymmetry of austenite peak occurs before the two-step isothermal process. The asymmetry is attributed to the development of carbon-rich and carbon-poor zones in austenite. The proportion of filmy austenite in austenite increases as the bainitic isothermal time prolonged. Moreover, the increase rate of carbon concentration in filmy untransformed austenite is large.